The primary objective is to measure the concentrations of mobile (diffusible) forms of seven physiologically important elements in muscle, and to compare the concentrations to the fixed (bound) forms of each element. A new method of sampling and analyzing cytoplasmic fluid by microdroplet equilibration and microdroplet X-ray spectroscopy will be used. Amphibian (normal) and mammalian (normal and dystrophic) muscles will be studied. The proposed experiments are designed to produce new information, previously unavailable, concerning the concentration, diffusivities and, to some extent, the molecular forms of the elements in the cytoplasmic fluid. This information is important for modeling the natural fluid environment of muscle protein systems. The specific objectives are: 1) To measure, by electron probe analysis of the liquid microdroplet samples, the intracellular concentrations of freely diffusible potassium, sodium, chloride, phosphorus, sulfur, calcium and magnesium. Conditions: near in situ; or following treatment of intact muscle with hyper-or hypo-tonic Ringer's solution, or irrigation of skinned fiber with simple salt solutions. 2) To measure, by cobalt marking and dilution methods, the extracellular and intracellullar fluid volumes of the muscles from which the liquid samples are taken. 3) To measure, by micro-ultrafiltration methods, the fraction of diffusible elements bound to large (0.5-20 kD) diffusible proteins. 4) To measure, by ion-selective microelectrode methods, the activities (and, from 1-3 above, the activity coefficients) of K+ and Cl- (and other ions, time permitting) in the microdroplets and in the whole muscle fiber. 5) To measure, as in 1 above, the concentrations and distributions of each intracellular element (total and diffusible) in dystrophic muscle fibers, and to compare the values with those from normal fibers.